1. Field of the Invention
The present invention pertains to improvements in cementing compositions and processes for cementing wells. The improvements provide better bonding and sealing of cement to adjacent surfaces such as casing and geologic formations. The improved seal provides better containment of fluids produced from or injected into geologic formations. The improved seal is also beneficial in containing fluids produced from or injected into the space between two casings where cement is used as a sealant at some point in the annulus between the casings.
2. Description of Prior Art
Subsequent to drilling a borehole of an oil or gas well, casing is run into the well and a cement slurry is placed in the annulus between the outside of the casing and the borehole wall. Typically, the cement slurry is placed in the annulus using a successive fluid displacement technique. In this technique, the cement is first pumped into the casing. Another fluid, often drilling fluid or water, is pumped behind the cement to displace it out the bottom of the casing and up into the annulus between the casing and borehole wall. The cement slurry is usually raised to a point above the uppermost section of the formations to be isolated and may be raised into the annulus between the casing string being cemented and a previously cemented casing.
Once placed, the purpose of the cement slurry is to harden in the annulus and provide an effective seal between the casing and the borehole. The hardened cement sheath prevents or inhibits communication of fluids between the various formations penetrated by the well. In addition to providing zonal isolation, the cement is in contact with the casing and provides mechanical support and resistance to axial movement through the shear bond developed between the interfaces of cement and pipe and cement and formation.
Bonding of the cement to the casing and borehole surfaces is critical to providing an effective seal in the annulus and for providing support for casings. Under most conditions, the bonding of cement to casing is achieved through contact of the cement particles with the surface of the casing. The resulting region of contact provides a mechanical interface which impedes movement of the casing due to high frictional forces. A fluid seal between cement and casing is effected by close contact of the cement particles at the casing surfaces which results in a region of very low effective permeability that prevents fluid migration along the interface.
Bonding between the cement and borehole wall is also achieved through contact of the cement particles with the formation or drilling fluid filter cake commonly deposited at the borehole wall during the drilling of the borehole. Some chemical combinations may also occur between the cement and formation or filter cake which may further enhance bonding and sealing at this interface. However, for this to occur the filter cake or formation should be water wet.
The present invention provides improved contact of the cement particles in an aqueous slurry at the interfaces of casing and formation or filter cake on the borehole wall. The improved contact is retained after the cement hardens resulting in a better fluid tight seal at the interfaces of cement and casing and cement and borehole wall. Better mechanical bonding between the cement sheath and casing and cement sheath and borehole wall is also realized.
The prior art describes various cement dispersants and retarders which are surface active agents; however, such agents are unlike the surface active agents of the present invention and do not provide the surface tension reduction and reduce shrinkage like the agents employed in the present invention. Napthalene sulfonate formaldehyde condensates, phenol sulfonate formaldehyde condensates, and lignosulfonates are commonly added to cement slurries to disperse the cement particles (U.S. Pat. Nos. 4,569,395; 4,687,516; 4,461,856; 3,132,693). These types of surface active agents serve the functions of 1) disaggregating the cement particle clusters and 2) preventing reaggregation of the dispersed cement particles in the slurry. Such surface active agents as these described in the prior art do not reduce the surface tension of the aqueous slurry to a value which improves the interfacial contact of the cement with adjacent surfaces.
Some surfactants of the type used in the present invention have been employed in the prior art for a different purpose than in the present invention, namely in the preparation of liquid fluid loss additives which may be added to the cement slurry (U.S. Pat. No. 4,687,516). In this patent, a dry polymer is slurried into a hydrocarbon carrier fluid. A surfactant is added to oil wet the dry polymer in the preparation. A second surfactant is added to water wet the polymer upon contact with water in the preparation of the cement slurry. While the type and concentration of surfactant in this prior art falls within the range of the present invention, the hydrocarbon carrier introduced prevents realization of improved sealing.
Other surfactants of the type used in the present invention also have been used in cement slurries for a different purpose than in the present invention, namely to prevent migration of gas through the matrix of the "unhardened", setting cement (U.S. Pat. No. 3,926,257). In this patent, cementing is effected by injecting a cement slurry containing a foaming agent which by cooperating with the gas of the formation produces a foam barrier which prevents any subsequent migration of gas through the cement slurry during the setting of the cement slurry.